In continuous ink jet printing, electrically conductive ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. Individual droplet streams are selectively charged in the region of the break off from the filaments and charge drops are deflected from their normal trajectories. The deflected drops may be caught and recirculated, and the undeflected drops allowed to proceed to a print medium.
Drops are charged by a charge plate having a plurality of charging electrodes along one edge, and a corresponding plurality of connecting leads along one surface. The edge of the charge plate having the charging electrodes is placed in close proximity to the break off point of the ink jet filaments, and charges applied to the leads to induce charges in the drops as they break off from the filaments. U.S. Pat. No. 4,560,991, issued Dec. 24, 1985, to W. Shutrum, describes one method of fabricating a charge plate. The charge plate taught by Shutrum is fabricated by electro-depositing the charging electrodes and leads on a flat sheet of etchable material, such as copper foil, to form a so-called "coupon." The coupon is bent in a jig at approximately a 90.degree. angle. The leads are then bonded to a charge plate substrate, and the etchable material is removed.
In the prior art, the charge plate substrate comprises an epoxy resin molded to completely surround the electrodes. This material is subjected to absorbing moisture, which can cause distortion of critical tolerances beyond their specifications. This distortion causes shifts in the positional relationship of the charge electrode to the ink droplet stream. This shift will induce a significant difference in charging current to the deflected droplet, causing an acceptable print head to perform poorly. Temperature changes can also adversely affect print quality. This distortion is magnified for long array ink jet printers exceeding one inch in length. In the prior art, ink jet printing systems are also susceptible to changes in temperature, which can adversely affect print quality.
It is seen then that there exists a need for a charge plate assembly having minimal susceptibility to dimensional changes during fabrication, and dimensional changes caused by environmental conditions.